Vessel provided with a foil situated below the waterline

ABSTRACT

Non-planing vessel with single hull ( 2 ). A foil ( 10 ) is fitted below the waterline at the place where the greatest flow constriction exists, in other words at the transition from the relatively broad part ( 9 ) to the narrowing part. This foil is situated at a distance from the hull ( 2 ) and preferably extends horizontally over as large a part of the hull as possible at a constant distance from the latter. The foil is designed in such a way that, apart from a lifting force, a forward propelling force is exerted. The performance of the vessel can be improved by fitting such a foil.

The present invention relates to a vessel with a single hull, the aftpart of which has a smaller water displacement and less breadth lying inthe water than the middle part, and provided with a foil extending belowthe waterline.

Such a vessel is known in the prior art. In particular, planing vesselsare sometimes provided near the end with a foil or flap extending belowthe hull, in order to ensure that during planing the trim angle that thehull forms with the surface of the water is limited. Such planingvessels generally have a hull that does not change substantially fromthe middle part in the aft direction.

An example of such a construction is found in U.S. Pat. Nos. 2,832,304;4,915,048 and the Abstract of Japanese Patent Application 62008893. Inthese cases the foil is situated on the aft side of the vessel, and adownward force is generated. Such vessels have no constriction of thesubmerged part of the vessel, in other words the cross section of thesubmerged part of the vessel (perpendicular to the direction of movementof the vessel) does not increase in the aft direction.

Furthermore, some catamarans are provided with fins for the purpose ofreducing ship movements in waves. Such fins do not, however, have a foilprofile. An example of the use of these fins in a boat is found in U.S.Pat. No 3,208,422.

The present invention relates to a non-planing vessel such as a sailingboat or a motor-driven vessel. It can be a relatively small vessel suchas a leisure vessel or a large vessel such as a tanker, container shipand the like. Such vessels are provided with a constriction, in otherwords the cross-sectional surface area decreases in the longitudinaldirection of the vessel, viewed in the aft direction. Such a reductionof the cross-sectional surface area is at least one third, and isnormally approximately 50% from the point with the largestcross-sectional surface area of the submerged part of the vessel to thepoint with the smallest cross-sectional surface area of the submergedpart of the vessel. At the position of a constriction the flow of thewater will be directed upwards and/or inwards.

More particularly, in the case of such vessels the transom is notsubmerged in the water, but lies above the waterline.

It is the object of the present invention to improve the sailingcharacteristics of such a vessel.

This aim is realised in a vessel of the type described above in thatsaid foil extends substantially transversely to the longitudinal axis ofsaid vessel, lying at a distance below the hull over a breadthcomprising at least half the breadth of said hull at the position of thegreatest flow constriction of said vessel caused by said reduction inwater displacement.

The terms length, breadth etc. are used in this description. These termsare associated with the position of the vessel in the water while it isbeing propelled in the usual direction. The direction of propulsion isthe longitudinal direction in this case.

According to the present invention, a foil (profile) is provided nearthe place where the greatest flow constriction occurs. This foil isarranged in such a way that, apart from an upward lifting force, aforward propulsion force is provided. This propulsion force manifestsitself by faster travel in the same conditions or by reduced fuelconsumption, or by both of these.

According to an advantageous embodiment of the invention, the distancefrom the foil to the hull is substantially constant over its fullbreadth. In other words, the foil follows the shape of the hullprecisely at a distance that does not vary.

The foil preferably extends over the greatest possible part of the“breadth” of the hull. In practice, this means that in the normalvertical position of the vessel the foil extends until close to thewaterline.

It will be understood that this does not necessarily have to be the casein relatively broad vessels such as some types of merchant vessels, forexample large oil tankers or container ships. The fact is that in thesecases the length of the foil below the hull is already such that thepart of it extending to the waterline would make only a smallcontribution to the effect described above.

The breadth of the foil is, however, important in the case of smallervessels such as sailing vessels.

Unlike the structure in the prior art, where the foil is used duringplaning and in that case can be moved between a position for planing andan ordinary sailing position, according to an advantageous embodiment ofthe invention, the foil according to the present invention is arrangedimmovably on the hull, and the distance from and orientation in relationto the hull is invariable.

According to an advantageous embodiment of the invention, the length ofthe foil, in other words the size in the longitudinal direction ofmovement of the vessel, is at least 50% of the distance from the hulland is no more than 150% of said distance.

The length of the profile decreases in the outward direction from thelongitudinal axis of the vessel. The rear edge preferably tapers off.

In experiments it was found that the overall hydrodynamic resistance ofa vessel can be reduced with the foil used above. It was also found thatthe stern wave of the vessel concerned is reduced, which results in afurther restriction of the resistance. These effects are greater thanthe resistance encountered by the foil through the water, so that a netpositive result remains. Furthermore, it is possible to limit the trimof a vessel with the foil.

The invention will be explained in greater detail below with referenceto an exemplary embodiment illustrated in the drawing, in which:

FIG. 1 shows diagrammatically in side view a vessel according to thepresent invention;

FIG. 2 shows the vessel according to FIG. 1 in top view; and

FIG. 3 shows the vessel in rear view.

The vessel according to the present invention is indicated in itsentirety by 1. It comprises a hull 2 which, as can be seen from FIG. 2,is provided with a fore part 3, which broadens out increasingly in thedirection of movement of the vessel. Connected to it is a middle part 7of substantially constant breadth, and following that a tapering aftpart 4. A considerable flow constriction occurs at the transitionbetween part 7 and part 4.

As can be seen from FIG. 1, the aft end of the vessel sometimes projectsabove the waterline.

The longitudinal axis of the vessel is indicated by 8. A foil is fittedunder the vessel. This foil is fixed in an immovable manner on thevessel by means of supports 11. The distance between the foil 10 and thehull 2 is constant and amounts to a. The length 1 at the position of theaxis 8 is preferably approximately equal to the distance a. The length 1of the foil decreases from the longitudinal axis 8 to the sides of thevessel, as can be seen from FIG. 2. The longest length is indicated by Lin FIG. 1, while the shortest length is indicated by L₁ in FIG. 2. Thisreduction of the length is obtained owing to the fact that the rear sideof the foil tapers. The breadth b of the foil is preferably at leastthree times, and more particularly at least four times, the length L ofsaid foil.

As can be seen from FIG. 1, the profile is of such a shape that itcauses a lifting force to be produced by the local flow rate at thatpoint. Owing to the fact that the local flow is directed obliquelyupwards (and backwards), this lifting force is not directed purelyvertically, but slightly forward. The resultants arising from this havea component in the forward direction.

The foil can be made of any material known in the art. This materialdepends on the application. For competition boats it will be a lightmaterial, such as a material impregnated with carbon fibres. In the caseof freight ships the material used will be steel or aluminium. Areduction in resistance of over 5% was found during experiments.

After reading the above, persons skilled in the art will immediatelythink of variants that are obvious after the above. These variants areconsidered to lie within the scope of the appended claims.

1. Vessel (1) with a single hull (2), the aft part (4) of which has asmaller water displacement and less breadth lying in the water than themiddle part (7), and provided with a foil (10) extending below thewaterline, characterized in that said foil (10) extends substantiallytransversely to the longitudinal axis (8) of said vessel, at theposition of the greatest flow constriction of said vessel caused by saidreducing water displacement, lying at a distance (a) below the hull overa breadth (b) comprising at least half the breadth of said hull at theposition of the greatest flow constriction of said vessel caused by saidreducing water displacement.
 2. Vessel according to claim 1, in whichthe distance of said foil from said hull is substantially constant overthe entire breadth.
 3. Vessel according to one of the preceding claims,in which said foil extends to the waterline (6).
 4. Vessel according toone of the preceding claims, in which the position of said foil relativeto said hull is fixed.
 5. Vessel according to one of the precedingclaims, in which the length (L) of said foil is at least 50% of thedistance (a) of said foil from the hull.
 6. Vessel according to one ofthe preceding claims, in which the length of said foil decreases in theoutward direction from the longitudinal axis (L₁).
 7. Vessel accordingto one of the preceding claims, in which said foil is designed in such away that during the forward movement of the vessel it exerts a forwardpropulsion force on said vessel.